1. Field of the Invention
The invention relates to a method of producing a gas discharge light source for emitting an electromagnetic radiation comprising a gas discharge tube which is filled with at least one discharge gas material, and a means for generating a gas discharge.
2. Description of the Prior Art
Such a method is known for example from U.S. Pat. No. 2,755,159. To ensure the suitable operating voltage of the gas discharge light source irrespective of the amount of discharge gas material (mercury) introduced and of variations of the dimensions, which both considerably influence the operating voltage, an excess of mercury is introduced and the excess then expelled again. A voltage source is connected to the gas discharge light source and the filled gas discharge tube ignited. The voltage present at the electrodes of the gas discharge tube is monitored, the voltage rising gradually until the entire mercury has evaporated and then starting to drop again when vapour from the overheated main section of the gas discharge tube reaches a colder side arm and condenses there. As soon as the voltage has dropped to a desired value the voltage source is switched off and the side arm serving as reservoir for excess discharge material is melted off. The steps described are intended to achieve that independently of said variations the operating voltage is fixed to 1 to 2 volts.
In U.S. Pat. No. 3,309,565 a fluorescence lamp is proposed in which in operation a temperature stabilization of a supply of condensed mercury takes place (a socalled "cold spot"). For this purpose, with the aid of a Peltier element the cold spot is cooled and the vapour pressure of the discharge material (mercury) held at a value which permits an optimum light emission of the gas discharge light source in operation.
In U.S. Pat. No. 4,431,947 a regulated light source is described which in operation makes available a light flux of constant intensity at a desired wavelength by employing a closed-loop control system which holds the intensity of a lamp generating light at a predetermined value. Such a light source of constant intensity is used for special purposes, for example in instruments operating with nuclear magnetic resonance, such as a magnetic resonance gyroscope. To achieve the desired constancy of the light at the desired wavelength, another wavelength is monitored and a control signal is derived therefrom which finally controls the temperature of a "cold end" of the light source.
In DE-PS 2,212,536 a method is proposed for producing a fluorescent lamp with which the aim is to ensure that the lamp bulb remains uninfluenced from impurities liberated on activation of the lamp cathodes. For this purpose the cathodes are activated outside the chamber in which the lamp bulb is filled, the impurities are removed and end caps with the cathodes are then sealed into the ends of the lamp bulb.
DE-OS 336,088 describes a production method for gas discharge lamps in which for removing impurities from the bulb interior the lamp is ignited and/or heated, a continuous stream of an inert ignitable gas being conducted through the bulb during the discharge and/or the heating. As flushing gas for example argon or another noble gas may be used.
In the prior art there has been no lack of further attempts to improve substantial parameters of such gas discharge light sources, for example the intensity of the emitted electromagnetic radiation, the stability and the like.
Thus, U.S. Pat. No. 4,157,485 has already proposed a low-pressure mercury vapour lamp in which the aim is to operate the mercury vapour lamp with maximum efficiency. For this purpose, it is considered necessary to keep the optimum mercury partial pressure of 6.times.10.sup.-3 Torr, corresponding to a temperature of the mercury of about 40.degree. C., as constant as possible. For if the lamp temperature rises above the ambient temperature at which the mercury in the discharge would have a temperature of about 40.degree. C. the efficiency drops and consequently the lamp power fluctuates with fluctuating ambient temperature. If the electrical power supplied to the lamp is increased the temperature rises and this in turn leads to a reduction of the conversion efficiency, i.e. the conversion of the electrical power supplied into electromagnetic radiation power. To ensure that the mercury partial pressure is stable over a wide temperature range at about the value 6.times.10.sup.-3, the use of an amalgam of bismuth, indium and mercury having a predetermined composition is proposed. This is said to be advantageous compared with already known amalgam compositions because in the known amalgam compositions the mercury proportion in the amalgam drops in operation because part of the mercury is bonded, in particular by absorption in a fluorescence layer.
A further improvement of the deposition mercury vapour lamp described in U.S. Pat. No. 4,157,485 has been proposed in EP-A1-0 157 440. In particular, with a higher discharge current and with a lamp tube of small diameter the stabilization value of the mercury vapour pressure with the amalgam proposed in the American patent specification is too low to obtain an optimum efficiency and maximum light output power. For at temperatures in the range of 100.degree.-120.degree. C. in the region of the amalgam the mercury pressure drops beneath 6.times.10.sup.-3 Torr. For this reason, EP-A1-0 157 440 proposes an alloy which forms an amalgam and consists of bismuth, lead and silver. This is intended to give an optimum light output power at a temperature of 70.degree.-150.degree. C. in the region of the amalgam. The alloy of bismuth, lead and silver is provided for this purpose at a socalled "cold spot" of the mercury vapour lamp at which a condensation takes place.
In U.S. Pat. No. 3,878,421 a high-power lamp for ultraviolet radiation is proposed which has a special electrode arrangement. The lamp tube here has a relatively large diameter and is provided with constricted end portions. Cold points, that is cold spots, form there in order to achieve partial condensation of the mercury vapour and thus prevent an excess vapour pressure of the mercury occurring which would reduce the lamp efficiency. The length of the end portions is greater or equal to twice the diameter of the end portions.
U.S. Pat. No. 4,005,332 describes a mercury fluorescence lamp which is operated with direct current. Whereas with alternating current operation an optimum efficiency is obtained at a mercury partial pressure of 7.times.10.sup.-3 Torr corresponding to a temperature of 40.degree. C. at the "cold spot" of the mercury, the exact values depending among other things on the lamp tube radius, the optimum temperature is obviously different in direct current operation. This optimum temperature depends on the poling in direct current operation and for a cold spot at the anode end of the gas discharge tube lies beneath 40.degree. C., the optimum temperature decreasing with increasing lamp current, whereas with an arrangement of the cold spot at the cathode end the optimum temperature is greater than 40.degree. C. and rises with increasing temperature. For this reason it is proposed in U.S. Pat. No. 4,006,332 to arrange the cold spot in a water bath, the temperature of which can be varied. In operation in this case enough mercury to ensure that a certain proportion thereof is always liquid is always present in the fluorescent lamp.
In U.S. Pat. No. 4,349,765 a mercury vapour lamp is proposed for ultraviolet radiation of high intensity in which mercury and inert gas are provided and the discharge takes place at a pressure of 5.times.10.sup.-3 to 5.times.10.sup.-1 Torr mercury and 5.times.10.sup.-2 to 10 Torr inert gas. The gas discharge tube consists of a quartz glass which is so doped that the lines of 185 nm and 194 nm of the emitted radiation are absorbed whereas the line of 254 nm passes through practically unattenuated. To set the optimum vapour pressure of the mercury a cold spot is again provided, i.e. a point at which the mercury is present in condensed, i.e. liquid form, and the temperature of the cold spot can be adjusted between 48.degree. and 65.degree. C. With this vapour pressure adjustment an intensity adjustment can be achieved.
An improvement of this ultraviolet gas discharge lamp is proposed in U.S. Pat. No. 4,835,442. To avoid fluctuations of the essential parameters of the gas discharge lamp a reservoir is proposed which serves as cold spot and the temperature of which can be adjusted. The reservoir is temperature-isolated with respect to the surroundings to eliminate said fluctuations.
An ultraviolet gas discharge lamp similar to that described in U.S. Pat. No. 4,349,765 is also disclosed in GB-PS 2,009,493. In CH-PS 570,040 a mercury vapour lamp is described as ultraviolet radiation source in which the gas discharge tube comprises a pressure equalizing space which connects the cathode space to the anode space. The sum of the volumes of the cathode space, the anode space and the pressure equalizing space is to be greater here than the volume of the discharge space. By a control member the mercury partial pressure is regulated so that the radiation yield in the region of 254 nm is at least 80% of the yield maximum. In this mercury vapour lamp as well a socalled cold spot is provided at which a drop of condensed, i.e. liquid, mercury is disposed.
Thus, in accordance with the prior art described above the desired improvement of the properties of gas discharge light sources is achieved by specific steps which fix the operation of said gas discharge light sources, i.e. the actual operating conditions.